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Start

A good place to start is the Chromium site. They have guides howto set your Chromebook into developer-mode and howto disassemble your device.

Both things will be nessary, if you want to flash your own version of Coreboot onto your Chromebook (in-system).
It is nessary to close a circuit on the motherboard via either a switch, jumper or srew to disable the write-protection of the spi chip. And to use the flashing tool flashrom in ChromeOs, you have to be in the developer-mode. If you already installed a Gnu/Linux-system or likely you can also use the flashrom from there (but then you would be in developer-mode anyway).

Note: The first three Chromebooks that were made are not supported by coreboot at the moment, but flashrom should be working. Mario, Alex and ZGB are running Insyde H2C some UEFI. Snow has das U-Boot nativly installed, but a coreboot port is available. All the others come shipped with Coreboot.

General Hardware Preperation

Use the disassembly guides to disassemble your Chromebook
till you see the place were you can put on a jumper, screw in a bolt or flip a switch. Check out the pictures in the disassembly guides to find the right spot. Either screw, switch or plug on now. For example in Lumpy and Stumpy you have to put a jumper on place between the Battery and Ram next to the Keyboard-Connector. Now assemble again (follow the guide). Now you are ready to flash your own Coreboot-version onto your Chromebook.

Building your own Coreboot-version and Payload

firmware chips

The firmware roms are highlighted on the pictures of the dissassembly guides:

Series 5 550 Chromebook (Lumpy) - Two firmware ROMs (one for EC, one for CPU+PCH) are located next to where the heatpipe meets the exhaust manifold. CPU+PCH firmware ROM is 8MB, EC firmware ROM is 256KB.

Samsung XE303CE ARM Chromebook (snow) - AP firmware ROM is located next next to USB 2.0 port. The EC firmware ROM is embedded in the EC itself and can only be accessed thru the mainboard debug port. AP firmware ROM is 4MB.

Lenovo X131E (Stout) - CPU+PCH actually has two firmware ROMs, but only one is used. The are located next to the DRAM slots on the bottom of the board. The one which is used is 8MB, the unused one is 4MB. The EC ROM is on the top-side next to the EC (underneath the keyboard, to the right of where the touchpad sits) and is 1MB.

HP Pavillion 14 (Butterfly) - CPU+PCH firmware ROM is on the bottom near the SD card slot and is 8MB. The EC firmware ROM is next to the EC, which is on the top side to the right of where the touchpad sits. The EC firmware ROM is 512KB.

Chromebook Pixel (Link) - CPU+PCH firmware ROM next to min-displayport connector. The EC firmware ROM is embedded in the EC itself and can only be access thru the mainboard debug port. CPU+PCH firmware ROM is 8MB.

Recovery/external programming

Ok, you fucked things up, but on the other hand this will give you the oppotunity to learn external chip-programming, so cheer-up.
First you will need to locate the SPI-Chip. If you done that you will need an external flashrom programmer (for example the open-hardware tool Bus Pirate). At the moment ask at the coreboot/flashrom mailing list for further details.